Immunization is one of the most significant public health achievements of the 20th century. Mass immunization has led to dramatic declines in vaccine-preventable diseases. Vaccines currently prevent more than 2 million deaths per year worldwide. However, an increasing number of parents are concerned about the safety of vaccines. Some of these concerned parents choose to either decline or delay immunizations for their children. Research has shown that parents who decline or delay immunizations place their children at greatly increased risk for pertussis, varicella, and measles infection. Research also demonstrates that vaccine-hesitant parents use the internet to obtain information on vaccines. Much of this information, however, is inaccurate and biased. Effective intervention strategies to reduce parental concerns about immunizations are needed. The objective of this proposed research study is to conduct a randomized intervention trial to measure the effectiveness of an interactive, social media website for parents who are concerned about vaccines. The website will feature various social media applications, including a blog, discussion forum and chat room. The website will also be a resource for providers who are interested in obtaining information about the newest vaccine-related concerns or discussing vaccine-related topics with parents. For this investigation, the website will be available to Kaiser Permanente Colorado (KPCO) patients and KPCO physicians. It will be moderated by physicians and vaccine researchers at the KPCO Institute for Health Research (IHR). At present, data on the relative benefit of using professionally moderated social media to influence health outcomes are lacking. If effective, this proposed intervention trial will represent an innovative, low cost and broadly applicable resource to reduce parental vaccination concerns.